Electrophoretic imaging process using transparent particles

ABSTRACT

AN IMAGE IS FORMED PHOTOELECTROPHORETICALLY BY EXPOSING AN IMAGING SUSPENSION TO A LIGHT IMAGE IN THE PRESENCE OF AN ELECTRIC FIELD. THE RESULTING IMAGE FORMED IS CONTACTED WITH A SECONDARY SUSPENSION CONTAINING AN ABRASIVE INGREDIENT IN TH PRESENCE OF SAID ELECTRIC FIELD AND EXPOSURE.

Jan. 1, 1974 v. TU LAGIN 3,782,932

ELECTROPHORETIC IMAGING PROCESS USING TRANSPARENT PARTICLES OriginalFiled 001;. 5, 1968 United States Patent O1 hoe 3,782,932 Patented Jan.1, 1974 Int. Cl. G03g 13/22 US. Cl. 965-13 4 Claims ABSTRACT OF THEDISCLOSURE An image is formed photoelectrophoretically by exposing animaging suspension to a light image in the presence of an electricfield. The resulting image formed is contacted with a secondarysuspension containing an abrasive ingredient in the presence of saidelectric field and exposure.

CROSS-REFERENCE TO RELATED APPLICATIONS This application is acontinuation of application Ser. No. 764,717, filed Oct. 3, 1968, nowabandoned.

BACKGROUND OF THE INVENTION This invention relates to an imaging systemand, more specifically, to an electrophoretic imaging system.

In photoelectrophoretic imaging colored photosensitive particles aresuspended in an insulating carrier liquid. This suspension is thenplaced between a pair of electrodes, subjected to a potential differenceand exposed to a light image. Ordinarily in carrying out the process theimaging suspension is placed on a transparent electrically conductiveplate in the form of a thin film and exposure is made through thetransparent plate while a second generally cylindrically shapedelectrode is rolled across the imaging suspension. The particles arebelieved to hear an initial charge when suspended in the liquid, whichcauses them to be attracted to the transparent base electrode and, uponexposure, to change polarities by exchanging charge with the baseelectrode such that the exposed particles then migrate away from thebase electrode to the roller electrode thereby forming images on both ofthe electrodes by particle subtraction, each image being complementaryone to the other. The process may be used to produce both polychromaticand monochromatic images. In the latter instance a single colorphotoresponsive particle may be used in the suspension or a number ofdifferently colored photoresponsive particles may be used all of whichrespond to the same Wavelength of light upon exposure. An extensive anddetailed description of the photoelectrophoretic imaging techniquedescribed above may be found in US. Pat. Nos. 3,384,565, 3,384,566, and3,383,993.

Although it has generally been found that good quality images can beproduced according to the above described procedures, due to the natureof the imaging suspension degrees of difficulty have been encountered inproducing high contrast images demonstrating the proper colorseparation. Various methods have been described whereby the overallcolor quality of both polychromatic and monochromatic imaging systemsare enhanced however none have yet to provide a technique whereby all ofthe above mentioned difiiculties have been eliminated.

an imaging system which will overcome the above noted disadvantages.

It is a further object of this invention to provide a high qualityelectrophoretic imaging system.

Yet, still a further object of this invention is to provide a highcontrast polychromatic and monochromatic imaging system.

Another object of this invention is to provide a novel process capableof producing high quality color images.

The foregoing objects and others are accomplished in accordance with thepresent invention generally speaking by providing an imaging suspensionof photoelectrophoretic imaging particles in an insulating carrierliquid. The suspension comprising the colored light absorbing particlesis placed in an electrode system containing at least one transparentelectrode and exposed selectively to actinic radiation while a voltageis applied by way of the electrodes across the imaging suspension. As aresult of particle migration within the system an image is formed on thesurface of the transparent electrode and unwanted particles migrate andadhere to the surface of the remaining electrode in the system. Thiselectrode supporting the unwanted particles is either removed from theconfiguration and replaced by still another similar electrode or thesurface is cleaned of the unwanted pigmented particles before continuingwith the process. The surface of the substituted or cleaned roller,whichever the case may be, is coated with a thin layer of a suspensionof substantially colorless or transparent particles in an insulatingcarrier liquid. The newly coated electrode is then contacted with theimaging suspension and the exposure step repeated, again applying avoltage across the imaging suspension. The latter image enhancement stepmay be repeated a number of times depending, for example, upon thedegree of color purity desired.

It has been determined that by treating the image formed according tothe above described process with a secondary dispersion or suspensioncomprising an insulating solvent having suspended therein a number ofsubstantially colorless or transparent particles that the overall colorquality and image contrast of the final image produced may besubstantially enhanced. It is hypothesized that as the result of theapparent. scrubbing or abrasive action of the suspended particles thereis produced a disturbance within the imaging zone during the re-exposurephase of the process. This abrasive action, it is speculated,contributes to the breaking up of particle aggregates and the apparentlayer structure of the suspension in a manner which substantiallyenhances the color separation and contrast of the final image.

The system of the present invention employs intensely colored pigmentparticles which serve both as the colorant and as the photosensitivematerial which apparently undergoes a net change in charge polarity uponexposure to activating radiation by interaction with one of theelectrodes. No additional photosensitive elements or materials arenecessary thus providing a very simple and inexpensive imagingtechnique. As a result of the mixture of two or more difi'erentlycolored pigment particles, each of which is sensitive only to light of aspecific wavelength, polychromatic images are produced. It has beenfound that the particles respond in the regions of the spectrum of theirprincipal light absorption with the cyan, magenta and yellow particlesresponding to red, green and blue light respectively. Thus the system ismost suited to subtractive color synthesis. Although the process of thepresent invention is especially suited for enhancing the colorseparation and image contrast of a polychromatic imaging system it islikewise suitable for the enhancement of the images resulting from amonochromatic system. In the latter instance a single colorphotoresponsive particle may be used in the imaging suspension or anumber of differently colored photo-responsive particles may be used allof which respond to the same wavelength of light exposure.

3 DESCRIPTION OF INVENTION The invention is further illustrated in theaccompanying drawing in which there is seen a transparent electrodegenerally designated 1 which, in this instance is made of a layer ofoptically transparent glass 2 overcoated with a thin layer of tin oxide3. Tin oxide coated glass of this nature is commercially available underthe trade name NESA glass. This electrode shall hereafter be referred toas the injecting electrode. Coated on the surface of the injectingelectrode is a thin layer 4 of the imaging suspension of the presentinvention comprising finely divided photosensitive pigment particlesdispersed in an insulating carrier liquid. The term photoresponsive forpurposes of this application refers to the properties of a particlewhich once attracted to the injecting electrode will migrate away fromthis electrode under the influence of an applied electric field whenexposed to activating radiation. A further detailed explanation of theapparent mechanism of this operation is disclosed in US. Pat. Nos.3,384,565, 3,384,566, and 3,383,993 herein incorporated by reference.

Above the liquid imaging suspension is passed a blocking electrodegenerally designated 5 which is represented herein as a roller electrodehaving a conductive central core 11 which is covered by a layer ofblocking electrode material 12. The core 11 is connected to a powersource 6 which in turn is connected by a switch 7 to the injectingelectrode 1. By closing switch 7 an electric field may then beestablished across the imaging suspension between electrodes 5 and 1.The pigment suspension is exposed by way of the projector mechanismconsisting of a light source 8, transparency 9, and a lens system 10.For purposes of this illustration a color transparency is used. Switch 7is closed and the blocking electrode 5 rolled across the upper surfaceof the injecting electrode 1. The light exposure causes the exposedparticles originally attracted to the injecting electrode to migratethrough the liquid carrier and adhere to the surface of the blockingelectrode material leaving behind a particulate image on the surface ofthe injecting electrode which is a duplicate of the transparency 9. Uponcompletion of the exposure step the pigment particles on the surface ofthe blocking electrode roller are removed by slowly rotating brush 14against the surface of the roller. As stated above in place of cleaningthe roller electrode a clean electrode may be substituted therefor. Ineither case the cleaned or new roller is coated with a thin layer of asuspension comprising an insulating carrier liquid having suspendedtherein the substantially colorless or transparent particles of thepresent invention. The roller electrode is then passed back across thesurface of the injecting electrode while again exposing the imagingsuspension in the same manner as discussed above. When the cycle iscomplete the blocking electrode roller may again be cleaned such as bybrush 13, or replaced as the case may be, and the entire cycle repeated.The cleanup brushes are merely exemplary of the various means whichmight be used to remove the migrated particles from the blockingelectrode surface. Any other suitable cleaning means may be used such aswiping, scraping, air blasting or using a spray solvent. The cycleincluding the image enhancement step may be repeated as many times asdesired. When the desired image is obtained it may be fixed such as byplacing a lamination over its surface or by virtue of the inclusion inthe carrier liquid of a dissolved binder material, such as parafiin wax,which will come out of solution as the carrier liquid evaporates. Sincethe carrier liquid may somewhat evaporate during the more extendedprocess a spray nozzle 15 may be provided to add additional carrierliquid between cycles to the imaging suspension. The additional carrierliquid will maintain the suspension in optimum conditions for particlemigration during the ensuing steps.

Other configurations may be used similar to that disclosed in theillustration and the configuration represented should be understood asmerely being illustrative of the present invention. For example, theroller type blocking electrode could use a replaceable web material orcontinuous belt which could be cleaned as discussed. Thus, a number ofways are available to execute the imaging steps herein described such ascleaning the unwanted particles from the electrode surface, replacingthe blocking electrode after each pass or just replacing the blockingelectrode material.

When used in the course of the present invention the term injectingelectrode should be understood to mean that it is an electrode which iscapable of exchanging charge with the photosensitive particles of theimaging suspension when the suspension is exposed to light thus allowingfor a net change in charge polarity of the par ticles. By the termblocking electrode is meant one which is capable of storing the electriccharge on its surface, thus it may be thought of as retarding theinjection of electric charges into the above mentioned photosensitiveparticles when the particles come into contact with the surface of theelectrode. It is preferred that the injecting electrode be composed ofan optically transparent material such as glass coated with a conductivematerial such as tin oxide, copper iodide, gold or the like; however,other suitable materials including many semiconductive materials whichare ordinarily not thought of as conductive but which are still capableof accepting injected charge carriers of the proper polarity under theinfluence of the applied field, may be used within the course of thepresent invention. The use of more conductive materials however allowsfor cleaner charge separation and prevents possible charge buildup onthe electrode, this characteristic tending to diminish the interiorelectrode field. The blocking electrode, on the other hand, is selectedso as to prevent or greatly retard the charge exchange of thephotosensitive pigment particles when the particles reach the surface ofthis electrode. The blocking electrode base generally will consist of amaterial which is fairly high in electrical conductivity. Typicalmaterials are conductive rubber, and various metal foils such as steel,aluminum, copper and brass. Preferably the core of the blockingelectrode will have a high electrical conductivity in order to establishthe required polarity differential. However, if a low conductivitymaterial is used a separate electrical connection may be made to theback of the blocking layer of the electrode. Although a blockingelectrode material may not necessarily be used in these systems, the useof such a material is preferred because of the markedly improved resultswhich it is capable of producing. It is preferred that the blockinglayer when used be either an insulator or a semiconductor which willprevent the passage of sufficient charge carriers under the influence ofthe applied field so as to prevent the discharge of those particlesbound to its surface thereby preventing particle oscillation within thesystem. Although the blocking electrode does allow for passage of somecharge carriers it would still be considered to come within the class ofpreferred materials if it does not allow for the passage of sufficientcarrier to recharge the majority of pigment particles to the oppositepolarity. Exemplary of the preferred blocking layer materials used arebaryta paper which consists of paper coated with a suspension of bariumsulfate in a gelatin solution, Tedlar, a polyvinyl fluoride, andpolyurethane. Other suitable materials having a resistivity of about 10ohm cm. or greater may be used as the blocking electrode material.Typical materials in this resistivity range include cellulose acetatecoated papers, polystyrene, polytetrafluoroethylene and polyethyleneterephthalate. The baryta paper, Tedlar and the other materials used asthe blocking layer may be wetted on their back surfaces with tap wateror coated with an electrically conductive material. The blockingelectrode layer, when utilized, may be a separate replaceable layerwhich is either taped to the blocking electrode core or held bymechanical fasteners or other devices which are capable of simplyholding the layer on the electrode. In the alternative, the layer may bean integral part of the electrode itself being either adhesively bonded,laminated, spray coated or otherwise applied to the surface of theelectrode.

Any suitable insulating carrier liquid may be used in the course of thepresent invention either as the carrier for the electrophoretic imagingparticles or the substantially transparent particle used in the imageenhancement step. Typical insulating carrier liquids include decane,dodecane, and tetradecane, molten paraflin wax, molten beeswax and othermolten thermoplastic materials, mineral oil, Sohio Odorless Solvent, akerosene fraction available from the Standard Oil Company of Ohio andIsopar G, a long chain saturated aliphatic hydrocarbon commerciallyavailable from the Humble Oil Company of New Jersey and mixturesthereof.

Any suitable ingredient may be used as the abrasive additive to thecleanup or image enhancement suspension of the present invention withthe additive generally being a solid, substantially colorless ortransparent organic or inorganic material. Typical particles utilized toprovide the effect desired include inorganic materials such as bariumsulfate, zinc oxide, silica dioxide, sodium chloride, barium titanate,and titanium dioxide, and organic materials such as pyrene, triphenols,phenanthrene, anthracene, tetracyanopyrene, tetrabromopyrene,tetranitropyrene, polyethylene, polypropylene, polyvinylchloride,polyethylene terephthalate, polyvinylfiuoride, polystyrene,polybutylmethacrylate, polytetrafiuoroethylene, 2,6-bis(2-hydroxy-3-tert-butyl-5-methyl-benzyl)-4 methylphenol, polyurethane,copolymers and mixtures thereof. Although not considered critical theparticle size of the added ingredient generally will be at least about0.10 microns or larger. The concentration of the particles present inthe cleaning suspension will vary depending upon the specific componentsutilized and the particles chosen. Generally, the concentration willrange from about 0.5 to about 25 percent by weight of the solventsuspension. Inasmuch as the added particle or ingredient is transparentor substantially colorless if the particles were to become a part of theresulting image, the presence of the particle would not interfere withthe color of the image produced.

A wide range of voltages may be applied between the electrodes in thesystem. For good image resolution, high image density and low backgroundit is preferred that the potential applied be such as to create anelectric field having a maximum value of at least about 300 volts acrossthe imaging suspension. The applied potential necessary to obtain thisfield of strength will, of course, vary depending upon theinterelectrode gap and upon the thickness and type of blocking materialused on the blocking electrode surface. For the very highest imagequalities the maximum field may be as large as 5000 volts. The upperlimit of field strength is limited only by the electrical conduction ofthe suspension and the dielectric properties of the blocking material.

In the polychromatic system, the particles are selected so that those ofdifferent colors respond to different wavelengths in the visiblespectrum corresponding to their principal absorption and further so thattheir spectral response curves do not have substantial overlap, thusallowing for color separation and subtractive multicolor imageformation. Several different particles are employed namely a cyancolored particle sensitive mainly to red light, a magenta coloredparticle sensitive primarily to green light and a yellow coloredparticle primarily sensitive to blue light. While this is the simplestcombination, additional particles having different absorption maxima maybe added to improve color synthesis. When mixed together in the carrierliquid, these particles produce a substantially black liquid and whenone or more of the particles are caused to migrate from the injectingelectrode to the blocking electrode they leave behind particles whichproduce a color equivalent to the color of the impinging light source.Thus, for example, red light exposure causes the cyan colored pigment tomigrate'thereby leaving behind the magenta and yellow pigments whichcombine to produce red in the final image. In the same manner blue andgreen colored light is reproduced by removal of yellow and magentapigment respectively and, of course, when white light impinges upon themix all pigments migrate leaving behind the color of the white ortransparent substrate. A dark exposure leaves behind all pigments whichcombine to produce a black image. It should be recognized that this isan ideal technique of subtractive color imaging in that the particlesare not only each c0mposed of but one component but in. addition theyperform a dual function in that they act both as the final imagecolorant and the photosensitive medium of the system. Accordingly, thesystem presents virtually the ultimate in eliminating the complexity ofprior art methods of subtractive the color imaging.

As stated above while the process of the present invention is especiallysuitable for enhancing the color separation and image contrast of apolychromatic imaging system it is likewise suitable for the enhancementof the images produced from a monochromatic system. In the latterinstance a single color photoresponsive particle may be used in theimaging suspension or a number of differently colored photoresponsiveparticles may be used all of which respond to the same wavelength oflight exposure.

It is desirable to use electrophoretic pigment particles which arerelatively small in size because smaller particles produce better andmore stable pigment dispersions in the liquid carrier and in additionare capable of producing images of higher resolution than is the casewith particles of larger sizes. Even where the pigments are notcommercially available in small particle sizes the particle size may bereduced by conventional techniques such as ball milling or the like.When the particles are suspended in the liquid carrier they may take ona net electrostatic charge and as a result may be attracted towards oneof the electrodes in the system depending on the polarity of the chargewith respect to that of the electrodes. It is not necessary that theparticles take on only one polarity of charge but instead particles ofboth polarities may be present. In such a case some of the particles inthe suspension will initially move towards the injecting electrode whileothers will move toward the blocking electrode; however, this particlemigration takes place uniformly over the entire area and the effect ofimagewise, exposure-induced migration is superimposed upon it. Thus, theapparent bipolarity of the suspension in no way affects the imagingcapability of the system except for the fact that it subtracts some ofthe particles uniformly from the imaging suspension before imagewisemodulation of the particles takes place. Thus a portion of the suspendedparticles are removed from the system as potential image formers. Thiseffect is overcome by merely forming an initial suspension ofsufiiciently' high particle concentration so that the system is stillcapable of producing the desired intense images. It has been found thatwith some of the suspensions described above either polarity ofpotential may be applied to the electrodes during imaging.

Typical photosensitive pigment particles which may be used to form thepigment mix of the present invention are disclosed in US. Pat. 3,384,488the disclosure of which is hereby incorporated by reference. Thepercentage of pigment in the insulating liquid carrier is generally notconsidered to be critical; however, for reference purposes it is notedfrom about 2 to about 10 percent pigment by weight has been found to behighly suited for the above disclosed process.

As stated above, ones the particle image has been formed it may be fixedto the respective electrode such as by spraying a binder onto thesurface, by laminating an overlay over the imaged surface, or byincluding a binder in the liquid suspension. Generally, it will bepreferable to transfer the image from the electrode and fix it on asecondary surface so that the electrode may be reused. Such a transferstep may be carried out by adhesive pickoff techniques, such as withadhesive tape or by electrostatic field transfer. In the latter instancea roller electrode is passed over the particle image on the injectingelectrode with the polarity of the potential applied being reversed fromthat originally applied during the imaging phase of the process. Thistransfer electrode may be covered with a paper sleeve which may beconveniently removed from the electrode core following image transfer.

Although various electrode spacings may be employed, spacings of lessthan about 1 mil and extending down to where the electrodes aresubstantially in virtual contact resulting from the electrodes beingpressed together are preferred. The latter condition constitutes aparticularly preferred form of the invention.

PREFERRED EMBODIMENTS the roller. A Tedlar sleeve is wrapped around theroller electrode to provide the blocking surface. The roller isapproximately 2 /2 inches in diameter and it is moved across the platessurface at about 1.4 cm. per second. The NESA plate employed is roughly3 inches square and is exposed with a light intensity of about 1800 footcandles. Exposure is made with a 3200 K. lamp through a Kodachromepositive transparency which is placed between the white light source andthe NESA glass substrate.

Example I An imaging suspension comprising equal amounts of .WatchungRed B, a barium salt of 1-(4'-methyl-5'-chloroazobenzene-2'-sulfonicacid)-2hydroxy-3-naphth0ic acid,

C.I. No. 15,865, monolite Fast Blue G.S., the alpha form of metal-freephthalocyanine, 0.1. No. 74,100, and a yellow pigment Algol Yellow G.C.,l,2,5,6-di(C,C'-diphenyl) thiazole anthraquinone, C.I. No. 67,300, inmineral oil "is prepared with the total pigment constituting about 8percent by weight of the suspension. The pigments are magenta, cyan, andyellow, respectively. The resulting mixture is coated on a NESA glasssubstrate and exposed as discussed above such that a positive polychromeimage is projected onto the tri-mix as the roller moves across thesurface of the NESA glass. The roller is held at a negative potential ofabout 2500 volts with respect to the NESA glass substrate. A suspensoncomprising a 10% solution of zinc oxide in mineral oil is coated on asecond roller similar to that used above and the second rollersubsequently passed over the NESA substrate 6 times under similarconditions. The roller surface is cleaned after each pass and the zincoxide suspension reapplied. After completion of the 6 passes it is foundthat an excellent quality full color positive image with all colors wellseparated is left behind on the NESA substrate. Potential applicationand exposure are both continued during the entire process.

Example H The process of Example I is repeated with the exception thatbarium sulfate is substituted for the zinc oxide. An image demonstratinggood color separation is produced.

8 Example III A process of Example I is repeated with the exception thattitanium dioxide is substituted for the zinc oxide. Similar results areobtained.

Example IV An imaging suspension comprising a metal-free phthalocyaninepigment, Monolite Fast Blue G.S., is prepared, 7 parts by weight of thephotosensitive particles being dispersed in a mineral oil carrier. Theresulting suspension is coated on a NESA glass substrate and theremainder of the process completed according to the procedure of ExampleI. The resulting image produced demonstrates the capabilities of thepresent invention in a monochrome imaging process.

Although the present examples were specific in terms of conditions andmaterials used, any of the above mentioned materials may be substitutedwhere applicable with similar results being realized. In addition to thestep used in the process of the present invention, other steps ormodificationsmay be used, if desirable. For example, the pigment used asthe additive in the suspension of the image enhancing step may bephotoconductive and thus this step in the process will also serve thepurpose of an additional imaging step. In addition, other materials maybe incorporated in the imaging suspension and other facets of theinvention which will enhance, synergize or otherwise desirably affectthe properties therein described. For example, various sensitizers maybe utilized in conjunction with the imaging suspension.

Anyone skilled in the art will have other modifications occur to thembased on the teachings of the present invention. These modifications areintended to be encompassed within the scope of this invention.

What is claimed is:

1. A photoelectrophoretic imaging process comprising:

(a) subjecting a layer of an imaging suspension to an applied electricfield between two electrodes, said suspension comprising a plurality offinely divided particles in a substantially insulating carrier liquideach of said particles comprising an electrically photosensitivepigment,

(b) substantially simultaneously exposing said suspen sion to an imagewith a source of electromagnetic radiation whereby an image is formed onan electrode and a complimentary image is formed on the other of saidelectrodes,

(c) replacing said other electrodes with an electrode having coated onits surface a suspension comprising transparent particles in asubstantially insulating carrier liquid, and

(d) re-exposing said imaging suspension while substantiallysimultaneously reapplying an electric field to the resultingconfiguration to improve said image.

2. The process as disclosed in claim 1 wherein the re-exposing of theimaging suspension in the presence of an electric field is repeated atleast once.

3. A photoelectrophoretic imaging process comprising:

(a) subjecting a layer of an imaging suspension to an applied electricfield between two electrodes, said suspension comprising a plurality ofat least two differently colored finely divided particles in asubstantially insulating carrier liquid each of said particlescomprising an electrically photosensitive pigment the principal lightabsorption band of which substantially coincides with its principalphotosensitive response and said pigment being the primary colorant forsaid particle,

(b) substantially simultaneously exposing said suspension to an imagewith a source of electromagnetic radiation whereby an image is formed onone electrode and a complimentary image is formed on the other of saidelectrodes,

(c) replacing said other electrode with an electrode having coated onits surface a suspension comprising transparest particles dispersed in asubstantially insulating carrier liquid, and

(d) re-exposing said imaging suspension while substantiallysimultaneously reapplying an electric field t0 the resultingconfiguration to improve said image.

4. The process as disclosed in claim 3 wherein steps (0) and (d) arerepeated at least once.

References Cited UNITED STATES PATENTS ROLAND E. MARTIN, JR., PrimaryExaminer 0 US. Cl. X.R.

